Antenna system



June 28, 1949. G. P. KEARSE 2,474,480

' ANTENNA SYSTEM Filed ma 18, 1948 mama June 2a. m

corpora poration of minoh Application my is. an. Serial No.

1 Claims. (on. mass) The present invention relates to antenna systemsand deals specifically with the problem of providing a televisionantenna designed'for high gain unidirectional performance throughout allof the thirteen television channels.

In introduction, it may be well to point out that throughout the historyof radio unccasing at titttw short of achieving the desired result, andit has tempts have been made to provide antennas cahas been greatlycomplicated by the advent of commercial television, primarily due to thefact that a broadcast signal covers only a few'kilocycles of the etherspectrum, while television reception from even a single station requiresan an antenna system adapted to respond to two separate signals, eachhaving a far greater spectrum width than a broadcast signal.

The magnitude of the task involved begins to become apparent when it isremembered that the videoor picture signal of a single televisionchannel covers approximately five hundred times the ether spectrum widthof a signal in the broadcast band, and the frequency modulation signalwhich supplies the sound to the television receiver is about fifteentimes as wide. These figures in themselves are staggering, but the fullsignificance of the problem is not appreciated until it is rememberedthat, to be successful from a commercial standpoint, a televisionantenna must not only respond to the signals within a single channel butmust respond with an equally high degree of performance to any of thethirteen channels presently in use. Further, these thirteen channels arenot continuous in the ether spectrum, since the first six channelsoccupy the frequency range of from 44 to 88 megacycles (with a breakfrom 72 to '76 megacycles. for non- Government fixed and mobileapplications), while the other seven channels function at a considerablyhigher frequency and occupy the range of from 174 megacycles to 216megacycles. The design of a successful television antenna presupposedthat it should reject other signals, yet the presence of the standardfrequency modulation band (88 to 108 megacycles) between theztwo groupsof television channels even further complicates the problem ofsatisfactory electrical design.

When the extreme range ofvfrequ'encies to which the antenna must respondis considered, it will be apparent that the conventional methods ofbroad banding an antenna stmcture, as by become more or less universalpractice to providea television receiver with a more or lessconventional dipole that is deliberately mismatched to the transmissionline. This gives a very poor gain at resonance, but the degree ofmismatching is greatly reduced on either side of resonance. with theresult that a fairly uniformresponse is achieved across a range offrequencies wide enough to reasonably cover one television channel (8megacycles) Such an arrangement, however, falls far short of offering afull solution to the problem, since it necessarily fails to produceanything even approaching high gain performance, is incapable ofoperation over the entire range of 44 to 216 megacycles and does'notyield'a bidirectional radiation pattern over all of this range. Inaddition, it is noted that even if satisfactory response over the entirespectrum could be achieved, a conventional antenna of this type wouldfail to reject the signals in the standard frequency modulation band,which could cause interference with the television signal.

It is accordingly the primary object of the present invention to providea television antenna system, wherein a single antenna array andconventional twin conductor transmission line may be employed for highgain performance throughout all of the present television channels, and

wherein the different components of the antenna are so designed andrelated as to achieve high gain performance, surpassing the performanceof even a resonant dipole and so designed as to discriminate againstfrequency modulation signals between the upper and lower groups oftelevision channels. l

A further object of the invention resides in the provision of an antennasystem having the performance characteristics noted above, yet having apronounced unidirectional pattern, extending substantially unchanged tothe full upper and lower limits of the present thirteen frequency bands.The importance of a unidirectional pattern in a television receivingantenna cannot be overstressed, since if the antenna tends to pick upsignals other than those emanating directly from the transmittingstation, the time interval between signals picked up directly and thoserefiected will cause reflected signals to produce multiple images orghosts, as they are called, and

lowering the Q of the circuit by decreasing the L/D (length to diameter)ratio fall hopelessly to-back and front-to-side ratio be extremely highto reject unwanted signals.

a further object of the invention is to provide 3 an antenna with theelectrical characteristics noted, yet so designed that it is inherentlymatched to a conventional 300 ohm twin lead transmission line withoutthe need of resorting to quarter-wave sections or other matchingdevices.

Theimportance of this feature isreadlly apparent when it is rememberedthat conventional receivers, as now manufactured, are almost invariablymatched to a 300 ohm line, and that useof any type of matching sectionbetween the line and the antenna necessarily causes the circuit tobecome frequency sensitiveand thusnullifies any I broad bandingcharacteristicslthat the, antenna itself might possess.

A further object, and one of considerable importance from a commercialstandpoint, resiciles" dipole Ill. 'The'dipole II and the reflector I9then function to provide an emcient unidirectionalarray At frequenciesin the upper six chanhels (174 megacycles to 216 megacycles) of the.

television spectrum, the dipole Ill functions as therefiector for thehigh frequency dipole l3, so

that here again an efficient unidirectional effect attained. Thus, bythe expedient of utilizing in the provision of an antenna having thefunctional characteristics heretofore discussed, and at the same timebeing of a. physical construction such that it is readily capable ofmounting on a single rotatable mast and designed in such a manner thatit may be commercially manufac taxed at reasonable. cost, and wheninstalled is rugged, durable, capable of withstanding high winds andheavy ice loading, yet having its total weight suiliciently 'low' thatit may safely be mounted on any reinforcing. The present inventionaccomplishes the objects noted by the provision of an antenna systemwhich uses, in combination, a folded dipole connected to the twin leadsof a conventional 300 ohm transmission line, together with a reflectorspaced from the dipole, and a second folded dipole positioned on theopposite side from the reflector and joined to the first by anadditional length of transmission line. The desired results heretoforeoutlined are achieved by'the simple expedient of designing the centerdipole for low frequency operation, and using a second dipole to therear of the low frequency dipole 28.

suited to operation over the upper group of fre-', quency channels; butcalculating the length of the transmission line and the second dipole soa that, at low frequencies, the circuit of the second dipole acts as aquarter-wave section, presenting substantially infinite impedance to thelow frequency signals and thus preventing the second dipole frominterfering with the performance of the first. The manner in which thisis accomplished is best illustrated with reference to the I drawing ofthis specification, wherein:

Figure 1 is a diagrammatic view of an antenna system as contemplated bythese teachings; and

Figure 2 is a perspective view or the preferred commercial embodiment ofthe invention as presently manufactured.

In the diagrammatic illustration of Figure 1 the dipole III is designedto operate over the low frequency group of channels and-is adapted to beconnected directly to the twin leads of the transmission line at thepoints II and I2. The high frequency dipole I3 is also of folded formand is designed to operate over the upper group of frequency channels.

The ends of the folded dipole it are also con-.. nectedto thetransmission line at the points II and I 2 by conductors II and I5, butthese conductors include looped sections l6 and I! so that they aresomewhat longer than the actual distance between the dipoles II and I3,and the length of the sections it and I1 is so calculated that theelectrical length between the points ll and I2 and the midpoint ll ofthe high frequency dipole is a quarter-wave length of the mean frequencyat which the low frequency dipole ll operates. Thus the impedance of theone folded dipole as a reflector for the other, it is not only possibleto attain high gain performance throughout the entire televisionspectrum, but it is also possible to accomplish unidirectionalcharacteristics through the entire frequency range by the use of onlythree elements. v In the commercial embodiment of the inventionillustrated in Flgure2, the entire array is mounted on a single fitting2| at the top of a mast 22. A cross'arm 23 extends both forwardly andrearwardly from the mast andcarries a back fitting 24 to support thereflector 25, and a front fitting 26 to support the high frequencydipole 27, The low frequency dipole 28 is mounted directly on thefitting 2|, and a twin lead 300 ohm transmission line 29 or any suitableradio frequency transmission line is joined to the ends of the dipole 28at the points 3| and 32. The pair of forwardly of the low frequencydipole.

conductors extending between these points and the second dipole is inthis case formed of an additional length of transmission line whichextends in a loose loop 23 to the ends 34 and 35 of the high frequencydipole. The transmission line is positioned parallel with the mast 22but spaced .,Best results are obtainedby providing a low frequencydipole 78 inches in length with a 3.25 inch center-to-center distancebetween itsupper and lower spans. Th'e reflector is 98 inches long andis spaced 38.5 inches behind the dipole 28. The high frequency dipole is29 inches long with 1.5 inch center-to-center distance between its upperand lower spans and is spaced 13.75 inches A 300 ohm twin leadtransmission line is utilized as the lead in, and it hasbeen learnedthat with the spacing and the dimensions of the parts given as above, itis unnecessary to form dependin sections, such as the sections l6 and II of Figure 1,

and that excellent results are achieved by merely allowing the sectionof line-33 to be suspended in a rather loose loop between the points 3l-32 and the points 31-", since in this particular construction theparts are so dimensioned that, at low frequencies, the electrical lengthbetween the points 3l-42 and the fitting 26 causes the circult of thedipole 21 to have almost infinite impedance.

Experiments with antenna systems constructed as'described above haveshown that they are not only efllcient from an electrical standpoint,but are also highly satisfactory by reason of their mechanical design.It ,will be readily seen that an antenna of this type may be easilyconstructed assessoiently assembled. It follows that it is well suitedto commercial sale, and may be installed in any upp r and lower limitsof operating frequencies, 3 and that the'front-to-back and front-to-sideratios are quite high, so that the system responds well to signalscoming directly from the transmitter but failsto respond to reflectedsignals andthus avoids ghost images on the screen of the l atransmission line; a reflector spaced from the television receiver.

The form of the invention illustrated and described herein is thepreferred embodiment of these teachings, in the form now commerciallymanufactured. It is shown as an illustration of the inventive concept,however, rather than by way of limitation, and it is pointed out thatvarious modifications and alterations may be indulged in within thescope of the appended claims.

Having thus described the invention, what 1 I claim as new and desireto-protect by United,

States Letters Patent is:

1. .In an antenna system, the combination, with transmission line, of afirst a radio frequency folded dipole directly joined to the ends of thetransmission line; a reflector spaced .from the first dipole at one sidethereof, and a second folded dipole shorter than the first and on theoppo- 1 site side of the first; and means for isolating said seconddipole from the first dipole at the resonant frequency of the firstdipole comprising a pair of conductors joining said seconddipole to thetransmission line; said conductors being of length greater than thespacing-between said first and second dipoles.

2. In an antenna system, the combination, with a radio" frequencytransmission line, of a first folded dipole directly joined to the endof the transmission line; a reflector spaced from the first dipole atone side thereof, and a second folded dipole shorter than the first andon the opposite side of the first; said second dipole also being joinedto the on line by a pair of conductors and the second folded dipole issubstantially equal to a shorted quarter wave length at the resonantfrequency of the first dipole.

5. In an antenna system, the combination, with a radio frequencytransmission line, of a first dipole. directly joined to the ends of thetransmission line: a second folded dipole at substantially the samehorizontal level and on one side of the first dipole and joined to thetransmission line by a pair of conductors of such length that the sum ofthe lengths. of the conductors and the second folded dipole issubstantially equal to a shorted quarter wave length line at theresonant frequency of the first dipole.

6. In an antenna system, in combination, an untuned radio frequencytransmission line, a first folded dipole directly joined to the ends ofthe first dipole at one side thereof and a second folded dipole shorterthan the first joined to the 1 transmission line by a pair of conductorsof such length that the sum of the lengths of the conductors and thesecond folded dipole is substantially equal to a shorted quarter wavelength line at the resonant frequency of the first dipole; the

'dipoles being at substantially the same horizontal level and spacedapart from each other where-- by one may act-as a reflector for theother.

7. A television antenna having a supporting frame consisting of avertical mast with a horizontal cross arm extending in oppositedirections from its upper end; a first folded dipole mounted across thetop of said frame, and a radio frequency transmission line directlyjoined to said first folded dipole: a reflector mounted on one end ofthe cross arm at substantially the same horizontal level as the firstdipole and spaced parallel with and at one side of said first foldeddipole;

a v second folded dipole shorter than the first mounted on the other endof the cross arm par- 1 allel with and at substantially the samehorizonfor isolating said second dipole from the first dipole at therwonant frequency of the first dipole comprising an additional looseloop of transmission line joining said second folded dipole to the online and the first folded dipole.

4. In an antenna system, the combination, with a radio frequency online, of a first dipole directly joined to the ends of the transmissionline; a reflector spaced from the first dipole at one side thereof, anda second folded dipole on the opposite side of the first dipole andjoined to the on line by a pair of conductors ofsuchlengththatthesmnofthelenithsoftho Number Name Date 2,039,295 CarterMay 5, 1936 2,255,520 Schuster Sept. 9, 1941 2,268,640 Brown Jan. 6,1942 2,297,329 -Scheldorf Sept. 29, 1942 2,352,977 scheldorf July 4,1944 2,452,073 Schivley et a1. e Oct. 26, 1948 FOREIGN PA'I'ENTS NumberCountry Date 520,628 Great Britain Apr. 80, 1940 7 OTHER REFERENCES 7Publication: FM 8; Television, Feb. 1948 p. 51,

tal level as the reflector and the first folded dipole and spaced fromsaid first folded dipole on the side opposite the reflector; and meansfor isolating the said second dipole from the first dipole at theresonant frequency of the first dipole comprising an additional looseloop of transmission line joining said second folded dipole to thetransmission line and the first folded dipole.

'- GEORGE P. KEARSE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS (see Collins antenna).

Publication: Amphenol Television Antenna Model 114-005, Manual file14.7, American Phenolic Corp., Chicago 50, Ill. March 1, 1948.

